It is known that carbon-carbon double bonds in polymers may be successfully hydrogenated by treating the polymer in an organic solution with hydrogen in the presence of a catalyst, as disclosed in U.S. Pat. Nos. 6,881,797, 6,683,136, 6,410,657, 6,020,439, 5,705,571, 5,057,581 and 3,454,644. Such processes can be selective in the double bonds which are hydrogenated so that, for example, the double bonds in aromatic or naphthenic groups are not hydrogenated and double or triple bonds between carbon and other atoms such as nitrogen or oxygen are not affected. This field of art contains many examples of catalysts suitable for such hydrogenations, including catalysts based on cobalt, nickel, rhodium, ruthenium, osmium, and palladium. The suitability of the catalyst depends on the extent of hydrogenation required, the rate of the hydrogenation reaction and the presence or absence of other groups, such as carboxyl and nitrile groups, in the polymers. As a characteristic, such hydrogenation processes always require the use of a large amount of organic solvent to dissolve the polymers to be hydrogenated and work at a relatively high temperature (larger than 100° C.), which raises concerns with respect to the production cost and environmental protection.
To eliminate the requirement for a large amount of organic solvents in hydrogenation operation, Laura R. Gilliom (Macromolecules Vol 22, no. 2, 1989, 662-665) and Laura R. Gilliom and Kevin G. Honnell (Macromolecules Vol. 25, no. 22, 1992, 6066-6068) had attempted to realize bulk hydrogenation of polymers using entrapped catalyst (Rh(PPh3)3Cl or [Ir(COD)(PMePh2)2]PF6). A conversion of about 90 percent was obtained at moderate temperatures and pressures; however, the reaction rate was very slow and the reaction needed five or more than five days to reach a relatively high conversion.
US-2006/0211826 A1 discloses a process for the selective hydrogenation of olefinic unsaturation in polymers and copolymers in bulk form without the addition of any organic solvent in the presence of hydrogen and a rhodium based catalyst. However, such process generally needs to be performed at relatively high temperatures of well above 100° C.
To milden the reaction conditions for the hydrogenation of unsaturated polymers, palladium-based catalysts for the hydrogenation of unsaturated polymer have received extensive attention:
U.S. Pat. No. 5,652,191 describes a catalyst precursor that is useful in the selective hydrogenation of olefinic unsaturation in polymers or copolymers in solution. The precursor comprises a palladium salt that is complexed with a complexing agent selected from the group consisting of organic phosphates, dialkylidene acetone, and tetraalkyl ammonium hydroxides.
U.S. Pat. No. 5,399,632 discloses a process for the hydrogenation of unsaturated homo- and copolymers in the absence of added gaseous hydrogen by contacting the unsaturated polymer with a hydrogen donor compound in the presence of a Group VIII metal catalyst, especially a supported palladium catalyst.
In the Journal of Polymer Science, Part A, Polymer Chemistry Vol 30, no. 3, 1992, 471-484 a homogeneous solution hydrogenation process using palladium acetate as a catalyst is described. A maximum conversion of 96% was reported.
U.S. Pat. No. 5,164,457 teaches a process for selectively hydrogenating a nitrile group-containing unsaturated copolymer in solution, in the presence of hydrogen and a palladium complex having a neutral ligand as a hydrogenation catalyst.
U.S. Pat. Nos. 4,892,928 and 4,876,314 disclose a process for selectively hydrogenating the ethylenic unsaturation in a polymer by contacting the polymer solution with hydrogen in the presence of a catalyst prepared by combining one or more palladium carboxylates with one or more aluminum compounds in a suitable solvent.
U.S. Pat. No. 4,510,293 describes a process for catalytically hydrogenating unsaturation of conjugated diene polymers by introducing hydrogen in the solution of said polymer, characterized in that a palladium salt of a carboxylic acid is used as a catalyst.
U.S. Pat. No. 5,272,202 teaches a process for selective hydrogenation of the C═C double bonds of unsaturated polymers in aqueous emulsion with hydrogen in the presence of a palladium compound as a catalyst and in an organic solvent capable of dissolving or swelling the polymer. Such organic solvent was used at a volume ratio of the aqueous emulsion to the organic solvent in a range of from 1:1 to 1:0.05.
U.S. Pat. Nos. 6,110,397, 6,063,307 and 5,837,158 disclose a process for removing hydrogen from the atmosphere within enclosed spaces by using a hydrogen getter. The hydrogen getter comprises organic polymer molecules having carbon-carbon double bounds, and a hydrogenation catalyst consisting of a noble metal catalyst such as palladium or platinum. Such hydrogenation catalyst is mandatorily supported on an inert catalyst support material. The C═C double bonds are hydrogenated which results in a removal of hydrogen. The processes of U.S. Pat. Nos. 6,110,397, 6,063,307 and 5,837,158, however, do not have the aim to provide maximum hydrogenation degrees but a viable way to remove undesired hydrogen traces from the environment.
Other heterogeneous palladium-based catalyst systems are described in U.S. Pat. Nos. 4,954,576, 4,853,441, 4,501,685 and 4,337,329. Each of the catalyst systems described in the said patents relates to a carrier and palladium supported thereon, which is used for the hydrogenation of olefinic unsaturation in diene-based polymers and copolymers, which were dissolved in an organic solvent.
In summary, the research in this area, namely, the hydrogenation of diene-based polymers, has been very successful if the polymers were dissolved in an organic solvent or if the hydrogenation is carried out at a relatively high reaction temperature. The research activities clearly focused on efficient hydrogenation of diene-based polymers in bulk form is so far very limited: In Macromolecules Vol 22, no. 2, 1989, 662-665 and Macromolecules Vol 25, no. 22, 1992, 6066-6068 rhodium and iridium based catalysts were applied, however, the reaction rate was very slow. In US-2006/0211826 A1 rhodium based catalysts were involved, however, relatively high reaction temperatures had to be applied.
The present invention therefore had the object to provide a new and improved process allowing the selective hydrogenation of a diene-based polymer in bulk form with a high degree of hydrogenation within acceptable short reaction time and at mild reaction temperatures.